The expression “MOS device” is used herein as a synonym for an MOS transistor.
The term “varactor” is used herein to denote a semiconductor device in which the electrical characteristic of primary interest is a voltage-dependent capacitance. For example, an NMOS (or PMOS) transistor can be employed as a varactor, with its capacitance determined (in part) by the voltage between its gate and source. For another example, a diode can be employed as a varactor.
The expression “exposure of a photodiode” (or “photodiode exposure”) is used herein to denote exposure of the photodiode to photons (to be sensed) during an exposure interval. The expression “subexposure interval” of an exposure interval (or exposure period) is used herein to denote a subinterval of the exposure interval (or exposure period). The expression “subexposure of a photodiode” (or “photodiode subexposure”) is used herein to denote exposure of the photodiode to photons (to be sensed) during a subinterval of an exposure interval.
The expression “active pixel sensor cell” is used herein to denote an image sensor that includes at least one active transistor. Typically, an active pixel sensor cell is implemented as an element of an array of identical sensor cells arranged in rows and columns. Typically, an active pixel sensor cell includes at least one photodiode, a reset transistor for each photodiode, and at least one other readout transistor (coupled to a column line) for reading out signals indicative of photogenerated charge that has accumulated on at least one terminal of the photodiode during an exposure or sequence of exposures. Each reset transistor is controlled to reset a photodiode. Typically for each exposure, at least one readout transistor is controlled to assert to a column line a pre-exposure signal indicative of voltage across the photodiode after the photodiode has been reset but before it has been exposed to the photons to be sensed, and then to assert to the column line a post-exposure signal indicative of voltage across the photodiode after the photodiode has been exposed to the photons to be sensed. The pre-exposure and post-exposure signals can be processed by readout circuitry (which is typically located along the column line far from the cell) to generate a signal indicative of photogenerated charge that has accumulated on at least one terminal of the photodiode during the exposure.
One type of conventional active pixel sensor cell is an MOS active pixel sensor cell that include at least one MOS transistor and at least one photodiode. In use, readout circuitry is coupled to the cell (e.g., to a column line coupled to the cell).
Traditional film-based cameras are rapidly being replaced by digital cameras that utilize an array of imaging cells (typically including a large number of imaging cells) to convert received light energy into electric signals indicative of an image. One type of imaging cell that is used in digital cameras to capture incident light energy is an active pixel sensor cell.
FIG. 1 is a schematic diagram of a conventional active pixel sensor cell 100. As shown in FIG. 1, cell 100 includes photodiode 112 (having a first terminal coupled to Node A and a second terminal that is grounded), NMOS reset transistor 114, whose source is connected to photodiode 112 and whose drain is maintained at potential Vdd, NMOS sense transistor 116 (a source follower amplifier transistor) whose gate is connected to photodiode 112 and whose drain is maintained at potential Vdd, and NMOS row select transistor 118 whose drain is connected to the source of sense transistor 116. The source of transistor 118 is coupled to a bitline. When, as is typical, cell 100 is an element of an array of cells arranged along rows and columns, the bit line is a column line. The gate of transistor 118 is coupled to receive a control bit “CTL.” Typically (e.g., when cell 100 is an element of an array of cells arranged along rows and columns), CTL is a row select bit that is pulsed high to select a row of sensor cells that includes cell 100.
Operation of active pixel sensor cell 100 typically includes three steps: a reset step in which transistor 114 is briefly pulsed on to place a predetermined initial voltage across photodiode 112; an exposure step in which photons incident on photodiode 112 are converted into electrical charge (i.e., photogenerated charge migrates to the first terminal of photodiode 112 while photodiode 112 is exposed to incident photons, thereby reducing the initial charge that has been placed on the first terminal during the reset step); and a signal readout step in which a signal indicative of the photogenerated charge is read out (i.e., as a current through the channels of transistors 116 and 118).
During the reset step, the gate of reset transistor 114 is pulsed with a reset voltage VR (e.g., VR=5 volts) to turn on transistor 114. In response, photodiode 112 is reset in the sense that it is charged to an initial voltage, VR−VT, between its terminals, where VT is the threshold voltage of reset transistor 114.
During the exposure step, photons that strike photodiode 112 create electron-hole pairs. Resulting photogenerated charge migrates to the terminals of photodiode 112. Photodiode 112 is designed to limit recombination between the newly formed electron-hole pairs. As a result, photogenerated holes are attracted to the second terminal (the grounded terminal) of photodiode 112, while photogenerated electrons are attached to the first terminal of photodiode 1112. Each photogenerated electron that reaches the first terminal reduces the voltage across photodiode 112.
At the end of the exposure step (sometimes referred to as the exposure interval), the final voltage across photodiode 112 is VR-VT-VS, where VS represents the change in voltage due to photogenerated carriers that reach the first terminal of photodiode 112. Thus, VS, which is indicative of the number of photons incident on the photodiode during the exposure interval, is determined by subtracting the voltage at the end of the exposure interval from the voltage at the beginning of the exposure interval: VS=((VR−VT)−(VR−VT−VS)).
Active pixel sensor cell 100 is read out by turning on row select transistor 118 (which is turned off during the reset and exposure steps) during a readout step after the exposure step. When row select transistor 118 is turned on, the voltage (VR−VT−VS) on photodiode 112 (Node A's potential above ground) determines the voltage on the gate of sense transistor 116 which, in turn, determines the magnitude of the current flowing through transistors 116 and 118. The current through the channels of transistors 116 and 118 is then detected by a conventional current detector (not shown) connected along the bit line (which is typically a column line).
One drawback of conventional active pixel sensor cells is that they typically operate poorly under low light conditions. With conventional film-based cameras, the amount of time that the shutter is open is adjustable over a wide range (e.g., from one thousandth of a second to capture an image of an object in motion, to several seconds to capture an image of an object under very low light conditions, such as at night).
With a conventional active pixel sensor cell, however, the maximum exposure interval in which a cell can be exposed to light energy is typically on the order of milliseconds. This is because a leakage current in the photodiode (known as dark current) can pull the initial voltage across the photodiode at the start of the exposure interval down to (or nearly to) zero in a time interval of this magnitude. Such leakage current is known as “dark current” because the leakage current can pull the initial photodiode voltage down to (or near to) zero even when no photons are incident on the photodiode.
Thus, when a conventional active pixel sensor cell is exposed to photons during an exposure period, the initial photodiode voltage falls in response to both the incident photons as well as the dark current. When the exposure period is sufficiently short, the dark current reduces the photodiode voltage by only a negligible amount relative to the amount by which photogenerated charge reduces the photodiode voltage.
However, when the exposure period is sufficiently long (e.g., on the order of milliseconds) incident photons cannot accurately be sensed because dark current during the exposure period reduces the photodiode voltage by a non-negligible amount relative to the amount by which photogenerated charge reduces the photodiode voltage. If (as is typical) an active pixel sensor cell cannot provide accurate results when operating with an exposure period of on the order of milliseconds (or longer), the sensor cell is not useful when operating in low light conditions.
There is a need for an imaging cell that can operate accurately with a longer exposure period than can conventional active pixel sensor cells, so that it can accurately sense incident radiation under low light conditions.